The present invention relates to the use of isocyanates for the chemical/thermal crosslinking of hot-melt pressure-sensitive adhesives (PSAS) based on non-thermoplastic elastomers, such as natural rubber, for example, using tackifying resins, optionally fillers and plasticizers, and to the application of these hot-melt PSAs to produce self-adhesive articles, especially for producing high-performance self-adhesive articles such as tapes or labels.
The hot-melt PSAs developed in recent years on the polymer basis of non-thermoplastic elastomers, such as, for example, natural rubber or other high molecular mass rubbers, in the absence of a crosslinking step after application lack sufficient cohesion for the majority of applications. This is manifested in inadequate shear strength of the self-adhesive tapes manufactured in this way and may even lead to the formation of disruptive residues of adhesive, which make it impossible to achieve a desired residueless redetachability after use.
For many years, this deficiency prevented the use of hot-melt PSAs based on natural rubber in the self-adhesive tape applications traditionally dominated heavily by natural rubber, such as masking tapes or adhesive tapes for packaging.
The crosslinking processes used to date for hot-melt PSAs based on non-thermoplastic elastomers, by means of ionizing radiation (electron beams=EBC or ultraviolet light=UV), require the presence of appropriate, cost-intensive installations such as radiation sources and complex protective equipment, especially at relatively high film thicknesses.
Furthermore, in the case of many customary ingredients such as fillers, non-transparent resins and pigments, and in the case of thick films of adhesive, UV crosslinking is possible only to an extremely limited extent.
The use of exclusively non-thermoplastic rubbers as an elastomer component in the formulation of PSAs with the existing cost advantage possessed, for example, by natural rubbers over the standard commercial block copolymers, and the outstanding properties, especially the shear strength of natural rubber and of corresponding synthetic rubbers, and also processes for preparing, applying and crosslinking hot-melt PSAs based on non-thermoplastic elastomers, are also set out at length in the patents WO 9411175 A1, WO 9525774 A1, WO 9707963 A1 and, correspondingly, U.S. Pat. No. 5,539,033, U.S. Pat. No. 5,550,175, and also EP 0 751 980 B1 and EP 0 668 819 B1.
In these cases, the additives customary in PSA technology, such as tackifier resins, plasticizers and fillers, are described.
The preparation process disclosed in each case is based on a twin-screw extruder which permits compounding to a homogeneous PSA blend with the chosen process regime, involving mastication of the rubber and subsequent gradual addition of the individual additives with an appropriate temperature regime.
The mastication step of the rubber, which precedes the actual production process, is described at length. It is necessary and characteristic of the process chosen, since with the technology selected therein it is indispensable to the subsequent integration of the other components and to the extrudability of the blended composition. Also described is the feeding-in of atmospheric oxygen, as recommended by R. Brzoskowski, J. L. and B. Kaivani in Kunststoffe 80 (8), (1990), p. 922 ff., in order to accelerate mastication of the rubber.
This procedure makes it absolutely necessary to practise the subsequent step of electron beam crosslinking (EBC), and to use reactive substances as EBC promoters in order to achieve an effective crosslinking yield.
Both process steps are described in the abovementioned patents, but the EBC promoters chosen also tend towards unwanted chemical crosslinking reactions at elevated temperatures, which limits the use of certain tackifying resins.
Owing to the unavoidable high product temperatures, compounding in a twin-screw extruder prevents the use of heat-activatable substances suitable for crosslinking the adhesive compositions, such as, for example, reactive (optionally halogenated) phenolic resins, sulphur or sulphur-donor crosslinker systems, since the chemical crosslinking reactions which ensue in the process result in such a great increase in viscosity that the coatability of the resulting pressure-sensitive adhesive composition is impaired.
The patent application JP 95 278 509 discloses a self-adhesive tape in whose production natural rubber is masticated to an average molecular weight Mw=100,000 to 500,000 in order to obtain a coatable homogeneous mixture comprising hydrocarbon resins, rosin/rosin-derivative resins or terpene resins, which can be processed regularly at between 140xc2x0 C. and 200xc2x0 C. with a coating viscosity of from 10 to 50xc3x97103 cps, but subsequently requires an extremely high EBC dose (40 Mrad) in order to ensure the shear strength necessary for its use.
For backing materials such as impregnated and/or sized papers, and for woven backings based on viscose staple and the like, the system is not very suitable, since at the necessarily high beam doses there is significant deterioration of the backing.
A disadvantage of the crosslinking technologies (essentially EBC irradiation) described in the documents cited, in addition to the capital investment required, is the damage of certain sensitive backings by electron beams. This is manifested to a particular extent in the case of paper backings, viscose staple wovens, and siliconized release papers, but especially in the case of widespread film materials such as polypropylene, by a deterioration in the elongation-at-break properties.
Moreover, many standard commercial PVC films tend to discolour under EBC irradiation, such discoloration having a deleterious effect in the case of light-coloured or transparent film grades.
Furthermore, many of the release coatings which are customary in adhesive tape manufacture are damaged by electron beam irradiation and so are impaired in their effect. In an extreme case, this may result in the non-unrollability of adhesive tape rolls or in the non-reusability of transfer release papers, which are required in the adhesive tape production process.
Certain synthetic rubbers such as polyisobutylene (PIB), butyl rubber (IIR) and halogenated butyl rubber (XIIR), finally, are not amenable to electron crosslinking and are degraded under irradiation.
One way of minimizing these disadvantages consists in the use of certain substances which lessen the required beam dose and thus the concomitant damage. A range of such substances are known for use as EBC promoters. However, EBC promoters may also tend towards unwanted chemical crosslinking reactions at elevated temperatures, which limits the selection of the EBC promoters that can be used for hot-melt PSA production and, moreover, restricts the use of certain tackifying resins. These restrictions, and certain advantageous combinations of EBC promoters and non-crosslinking phenolic tackifier resins, are the subject-matter, in particular, of the document WO 97/07963.
The use of non-thermoplastic elastomers is also described in JP 95 331 197, where use is made of natural rubber having an average molecular weight (weight average) Mw less than 1 million g/mol with aliphatic, non-reactive hydrocarbon resins, which is blended with blocked isocyanates, precrosslinked at 150xc2x0 C. for five minutes, and, following its subsequent coating onto PET film, is cured at 180xc2x0 C. for several minutes (for example 15 minutes).
A disadvantage of this process, firstly, is the blocking agent released during the crosslinking reaction, which on the one hand, if it remains in the adhesive composition, may impair the adhesion properties of the tape in a variety of respects, and on the other hand, on its escape in vapour form, leads to coating defects such as porosities and necessitates complex technology in order to draw off these blocking agents under suction and remove them.
Of particular disadvantage, however, is the high crosslinking temperature, which rules out temperature-sensitive backings such as many films and foams on principle and in the case of paper backings and release papers may lead to embrittlement.
In summary it may be stated that crosslinking of the known hot-melt pressure-sensitive adhesives based on non-thermoplastic elastomers necessitates either damagingly high radiation doses or else damagingly high temperatures with long crosslinking times, and both have the consequence of damage in the case of a large number of the customary backing materials.
The object of the invention is to remedy this situation and to combine the economic advantages of the solvent-free manufacture and application of hot-melt pressure-sensitive adhesives based on non-thermoplastic elastomers with the chemical-thermal crosslinking possibilities of the conventional solvent technology for thick adhesive compositions, including filled and coloured adhesive compositions, with high film thicknesses on radiation- and temperature-sensitive backing materials.
This object is achieved by means of a hot-melt pressure-sensitive adhesive as characterized more closely in the main claim. The subsidiary claims relate to advantageous developments of the subject-matter of the invention, to advantageous possibilities for use, and to processes for producing the backing materials coated with the subject-matter of the invention.
The invention accordingly provides a hot-melt pressure-sensitive adhesive (PSA) based on one or more non-thermoplastic elastomers which comprises:
100 parts by mass of the non-thermoplastic elastomer(s),
from 1 to 200 parts by mass of one or more tackifying resins and also one or more polyfunctional isocyanates which are free from blocking agent, the hot-melt PSA comprising from 8 mmol to 5 mol of the reactive isocyanate groups of the isocyanate per kilogram of the non-thermoplastic elastomer(s) used.
Preferably, the elastomer or elastomer mixture has an average molar mass of from 300,000 to 1.5*106 g/mol, determined as the weight average using a GPC measurement. In the GPC measurement (gel permeation chromatography, a liquid chromatography conducted in the form of a column chromatography) a liquid phase comprising the dissolved polymer is passed through a gel. Smaller molecules of the dissolved substance are able to penetrate (diffuse) into all the pores; to them, the entire volume of the mobile phase in the separating column is available. For this reason, they are retained longer in the column than are the larger molecules. These molecules, which are larger than the largest pores of the swollen gel, are unable to penetrate the gel particles and migrate past them; they leave the column first. Consequently, in the eluate, the molecules appear in the order of decreasing molecular size. Since the molecular size is generally proportional to the molar mass, gel chromatography offers the possibility of separating and purifying substances of different molar masses and of determining molar masses.
With further preference, the hot-melt PSA in the uncrosslinked state has a complex viscosity of from 10,000 to 300,000 Pa*s at 0.1 rad/s and 110xc2x0 C., preferably from 30,000 to 170,000 Pa*s at 0.1 rad/s and 110 C, very particularly from 40,000 to 140,000 Pa*s at 0.1 rad/s and 110xc2x0 C.
The non-thermoplastic elastomers are advantageously selected from the following group, either individually or in any desired mixture:
natural rubbers
random-copolymerized styrene-butadiene rubbers (SBR)
butadiene rubbers (BR)
synthetic polyisoprenes (IR)
butyl rubbers (IIR)
halogenated butyl rubbers (XIIR)
ethylene-vinyl acetate copolymers (EVA)
polyurethanes.
In a further advantageous development of the invention, the hot-melt PSA comprises a polymer blend of one or more of the non-thermoplastic elastomers and one or more thermoplastic elastomers, the latter again being selectable from the subsequent listing, individually or in any desired mixture:
polypropylenes
polyethylenes
metallocene-catalysed polyolefins
polyesters
polystyrenes
synthetic block copolymer rubbers
As reactive non-blocked isocyanates it is possible to use both aliphatic and aromatic isocyanates that are free from blocking agent, it also being possible for the blocking-agent-free isocyanate to comprise a mixture of different isocyanates which are distinguished by different reactivities.
Isocyanates which may be used include the following, in a list which should not be understood as being conclusive:
toluene diisocyanate (TDI)
diphenylmethane diisocyanate (MDI)
hexamethylene diisocyanate (HDI)
isophorone diisocyanate (IPDI)
bis(4-isocyanatocyclohexyl)methane (PICM/H12*MDI)
By way of example, mention may also be made of the following:
Additive TI, Bayer AG (toluenesulphonyl-NCO)
Desmodur CD, Bayer AG
Vestanat IPDI, Hxc3xcls AG (isophoronedi-NCO)
Vestanat T 1890/100, Hxc3xcls AG (poly-NCO)
TMI, Cytec Industries B.V
Voranate M 220, Dow Chemical Europe (diphenylmethane diisocyanate)
Desmodur RFE, Bayer AG
TMXDI (meta), Cytec Industries B.V (TMXDI)
Desmodur TT, Bayer AG (TDI)
The crosslinking reaction may take place in the usual manner on the open belt in tunnel installations with an appropriate temperature regime.
For the crosslinking of the hot-melt PSAs it is also possible to utilize the heat treatment processes which are often used in adhesive tape production and are as required, for example, for the relaxation of film materials, or crosslinking may be effected at room temperature on the belt. In one particularly advantageous embodiment, the crosslinking is controlled not only by means of temperature and residence time but also by the addition of catalysts, as are known for PU chemistry and are described, inter alia, in xe2x80x9cCatalysis in aliphatic isocyanate alcohol reactionsxe2x80x9d [xe2x80x9cModern paint coatingxe2x80x9d Vol. 77 1987 No. 6 page 28-32 and p. 35].
In all cases, there is an absence of damage to the backing by high radiation doses or by high crosslinking temperatures, and of defects in the application pattern or function of the hot-melt PSAs crosslinked in this way.
In order to induce an acceleration in the crosslinking, the said blocking-agent-free isocyanate of the hot-melt PSA may be admixed with a catalyst, such as dibutyltin laurate, for example.
In principle, a large number of further catalysts are suitable. The following table represents one possible selection of catalysts, and should not be regarded as being conclusive. Rather, the person skilled in the art will be confronted with no difficulties whatsoever in using substances other than those explicitly mentioned here.
Octoates (a non-systematic designation for the octanoates (octanates, caprylates)) are the esters and salts of octanoic acid. In the art, however, the term octoate is understood generally to refer to the metal salts of 2-ethylhexanoic acid. The octoates used as metal soaps and driers are low in odour, easier to control than, for example, the naphthenates, and also accelerate drying in a humid atmosphere. Siccatives used are, for example, Ca, Zn, Mn, Co and Pb octoates. Ba octoate is a stabilizer for PVC. Li, Mg, Ca and Al octoates are used to produce mineral oil lubricants. Al octoate is also suitable for gelling light hydrocarbons for cosmetics. Ni octoate and Cu octoate are recommended as oil-soluble fungicides, Ca octoate as a reference substance in atomic absorption spectroscopy, Fe octoate as a combustion catalyst for liquid fuels, Na and K octoates in the preparation of water-soluble penicillins (source: Rxc3x6mpp Lexikon Chemie-Version 1.5, Stuttgart/New York: Georg Thieme Verlag 1998).
To the hot-melt PSA it is possible to add fillers, which may in particular be selected from the group consisting of metal oxides, chalks, with particular preference chalks having specific surface areas of from 3 to 20 m2/g, precipitated or pyrogenic silicas, with particular preference silicas having specific surface areas of from 20 to 250 m2/g, preferably from 40 to 200 m2/g, solid or hollow glass beads, with particular preference the solid or hollow glass beads having an average diameter of from 3 to 200 xcexcm, preferably from 5 to 135 xcexcm, microballoons, carbon blacks, with particular preference carbon blacks having specific surface areas of from 20 to 120 m2/g, and/or glass fibres or polymer fibres.
It is also possible for the surface-modified variants of the fillers recited above to find application.
The microballoons are elastic, thermoplastic hollow beads which have a polymer shell. These beads are filled with low-boiling liquids or with liquefied gas. Suitable shell polymers are, in particular, acrylonitrile, PVDC, PVC or acrylates. Hydrocarbons such as the lower alkanes, pentane, for example, are suitable as the low-boiling liquid, while a suitable liquefied gas is a chemical such as isobutane.
Particularly advantageous properties are manifested when the microballoons have a diameter at 25xc2x0 C. of from 3 xcexcm to 40 xcexcm, in particular from 5 xcexcm to 20 xcexcm.
By exposure to heat, the capsules expand irreversibly and three-dimensionally. Expansion comes to an end when the internal pressure is equal to the external pressure. In this way, a closed-cell foam backing is obtained which features good flow behaviour and high recovery forces.
Following thermal expansion due to elevated temperature, the microballoons advantageously have a diameter of from 20 xcexcm to 200 xcexcm, in particular from 40 xcexcm to 100 xcexcm.
This expansion may take place prior to or following the incorporation into the polymer matrix, or else before or after incorporation into the polymer matrix and shaping.
It is also possible to perform the expansion following incorporation into the polymer matrix and prior to shaping.
The fillers should be added individually or in any desired combination in proportions of from 1 to 100 per 100 parts of elastomer.
The fillers should be added individually or in any desired combination in proportions of from 1 to 300 per 100 parts of elastomer.
It is further advantageous to admix plasticizers into the hot-melt PSA, the said plasticizers in turn being selected in particular from the group consisting of paraffinic or naphthenic oils, with particular preference paraffinic or naphthenic oils having kinematic viscosities at 20xc2x0 C. of between 40 and 255 mm2/s, oligomeric nitrile rubbers, with particular preference liquid nitrile rubbers having ACN contents of from 20 to 40% by weight, in particular from 20 to 35% by weight, liquid isoprene rubbers, with particular preference isoprene rubbers having molar masses of between 10,000 and 70,000 g/mol, oligobutadienes, with particular preference oligobutadienes or functionalized oligobutadienes having molar masses of from 1500 to 70,000 g/mol, soft resins, with particular preference soft resins having molar masses of from 100 to 2000 g/mol, in particular from 250 to 1700 g/mol, wool fats and/or rapeseed oils and castor oils.
The hot-melt PSA of the invention may find application in the production of a self-adhesive article by application of the said adhesive to at least one side of a web-form material, for example a material coated anti-adhesively on both sides, the hot-melt PSA being applied with a preferred rate of from 5 to 3000 g/m2, with particular preference from 10 to 200 g/m.
The web-form material may in particular comprise a single-sidedly or double-sidedly coated paper backing or a single-sidedly or double-sidedly coated polymer film backing, in which case the application rate may be from 5 to 200 g/m2 and in particular from 10 to 100 g/m2.
Surprisingly it is possible, relative to the hot-melt PSA compositions crosslinked with the known radiation techniques, to find a significant improvement in the anchoring of the hot-melt PSA compositions, isocyanate-crosslinked in accordance with the invention, to standard commercial unprimed films, especially when the films concerned are comparatively non-polar films such as polypropylene or polyethylene, for example, and when these films have been pretreated chemically by means of standard commercial primers or physically by means of customary flaming techniques or corona treatment.
As backings it is further possible to use wovens or nonwovens of all kinds.
Consolidated nonwoven webs are produced, for example, on stichbonding machines of the xe2x80x9cMalifleecexe2x80x9d type from the company Malimo and can be obtained, inter alia, from the companies Naue Fasertechnik and Techtex GmbH. A Malifleece is characterized in that a cross-laid web is consolidated by the formation of loops from fibres of the web.
The backing used may also be a web of the Kunit or Multiknit type. A Kunit web is characterized in that it originates from the processing of a longitudinally oriented fibre web to form a sheetlike structure which has the heads and legs of loops on one side and, on the other, loop feet or pile fibre folds, but possesses neither filaments nor prefabricated sheet-structures. A web of this kind has also been produced for many years, for example, on stitchbonding machines of the xe2x80x9cKunitvliesxe2x80x9d type from the company Karl Mayer, formerly Malimo. A further characterizing feature of this web is that, as a longitudinal-fibre web, it is able to absorb high tensile forces in the lengthwise direction. The characteristic feature of a Multiknit web relative to the Kunit is that the web is consolidated on both the top and bottom sides by virtue of the double-sided needle punching.
Finally, stitchbonded webs are also suitable. A stitchbonded web is formed from a nonwoven material having a large number of stitches extending parallel to one another. These stitches are formed by the incorporation, by stitching or knitting, of textile filaments. For this type of web, stitchbonding machines of the type xe2x80x98Maliwattxe2x80x99 from the company Karl Mayer, formerly Malimo, are known.
Starting materials envisaged for the textile backing are, in particular, polyester fibres, polypropylene fibres or cotton fibres. The present invention, however, is not restricted to the aforementioned materials; rather, a large number of other fibres may be used to produce the web.
Needlepunched, wet-laid and/or air-jet- and/or water-jet-consolidated webs may be obtained, for example, from the company Freudenberg.
Particularly suitable thicknesses which have been found for the hot-melt PSA on the web-form material are between 5 xcexcm and 3000 xcexcm, preferably between 15 xcexcm and 150 xcexcm.
Furthermore, the hot-melt PSA may have been applied to a double-sidedly anti-adhesively coated release paper in a thickness of from 20 xcexcm to 3000 xcexcm, in particular from 40 xcexcm to 1500 xcexcm.
One particularly suitable process for producing the self-adhesive articles set out above, especially for producing high-performance self-adhesive articles such as tapes or labels, is to apply the hot-melt PSA using a multi-roll applicator unit comprising from two to five rolls.